It has been estimated that approximately 4% of the people in the world suffer from depression which is not caused by any underlying neurological disease. Depression effects people in all walks of society, from the very young to the very old. It often occurs without the presence of a precipitating event, and is frequently unresponsive to psychotherapy or environmental changes.
The cluster of symptoms associated with depression suggests that it is caused by a defect which affects the regulation of neurotransmitters. Neurotransmitters are substances that are synthesized and released synaptically by one neuron and which effect a postsynaptic cell through a specific receptor. The major small molecule transmitter substances include acetylcholine, the biogenic amines such as dopamine, norepinephrine, serotonin and histamine, as well as amino acids such as glutamate. Neurons also communicate with other neurons or target cells through the neuroactive peptides, which include somatostatin and (.beta.-endorphin. Neuronal cells often produce a combination of small molecule transmitters and neuroactive peptides at their synapses.
Present theory postulates that a deficiency in the neurotransmitters serotonin and/or norepinephrine, or both, results in the symptoms of depression. Norepinephrine is synthesized from tyrosine through the enzymes tyrosine hydroxylase, which converts tyrosine to L-DOPA, which in turn is acted on by a decarboxylase to give dopamine, which is then acted on by dopamine .beta.-hydroxylase, converting dopamine to norepinephrine. Serotonin is synthesized from tryptophan through the action of the enzymes tryptophan hydroxylase, which oxidizes tryptophan to 5 hydroxytryptophan (5-OH-tryptophan), and the enzyme 5-OH-tryptophan decarboxylase, which converts 5-OH-tryptophan to serotonin (5 hydroxytryptamine; 5HT). Once synthesized by a neuronal cell, serotonin and norepinephrine are stored within the cell in vesicles.
In the central nervous system (CNS), norepinephrine-containing nerve cell bodies are present in the locus ceruleus (among other areas) which projects throughout the cortex, cerebellum and spinal cord. In the peripheral nervous system, the postganglionic neurons of the sympathetic nervous system use norepinephrine as a neurotransmitter.
Serotonin containing cell bodies are present in the midline raphe nuclei, which projects throughout the brain and spinal cord. Multiple families of serotonin receptors exist in the CNS, including the 5-HT.sub.1 (subtypes 5-HT.sub.1A, 5-HT.sub.1B and 5-HT.sub.1C, and 5-HT.sub.1D) as well as 5-HT.sub.2, 5-HT .sub.3 and 5-HT.sub.4 families of receptors. Each 5-HT receptor is associated with an individual pharmacology for agonists and antagonists. (Frazer A., et al. Annual Rev. Pharmacol. Toxicol. 1990; 30: 307-348; Bockaert J., et al. Mol. Pharmacol 1990; 37: 408-411). For example, 5-HT.sub.1A agonists are potent antidepressants.
When norepinephrine or serotonin containing neurons are stimulated, these transmitters are released to act transiently on effector cells or organs, to alter the conductance of ions through channels of the postsynaptic membrane, or to alter the biochemical activity within the cell. Removal of neurotransmitters from the synaptic cleft may entail breakdown by cytoplasmic enzymes or reuptake into vesicles in presynaptic neurons.
The theory that depression is associated with a regulatory imbalance involving the biogenic amines serotonin and norephinephrin originated with studies involving reserpine. This compound, which can be used to induce depression, interferes with the granular storage of biogenic amines in presynaptic neurons, thus causing their release and eventual depletion.
Two therapies are generally used to treat depression, which may have a mode of action which alters, at least temporarily, regulatory imbalances in the biogenic amines. The first is electroconvulsive therapy (ECT), which, in effect, causes the induction of a generalized seizure. ECT has been found to cause an improvement in approximately 90% of patients on which it is used.
Antidepressant drugs include the monoamine oxidase inhibitors and the tricyclic compounds, both of which are effective in about 70% of the cases in which they are used. The monoamine oxidase inhibitors prevent the degradation of cytoplasmic serotonin and norepinephrine. The tricyclic compounds block the active reuptake of serotonin and norepinephrine by neurons, thus permitting these transmitters a longer time to act in the synaptic cleft. In addition, serotonin precursors such as tryptophan and 5 HT act as antidepressants.
In addition to a serotonergic mechanism, there is some evidence that drugs acting via a dopaminergic mechanism may have antidepressant activity. Tricyclic antidepressants appear to affect the dopaminergic system at the presynaptic level by enhancing the release of dopamine. Chiodo, L. A. et al. 1980; Eur. J. Pharmacol 63:203-204.
Thus, although the specific cause of depression has not been fully elucidated, the biogenic amines appear to play a significant role in the process. It appears, therefore, that agents that alter the intracellular and extracellular levels of these compounds in the brain would be expected to have some effect on depression.
One group of compounds that have recently been found to be active in altering serotonin turnover in the brain are the neurotrophins. The neurotrophin family includes brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4), all of which have recently been molecularly cloned and shown to be members of the nerve growth factor (NGF) family by virtue of their sequence homology [Leibrock, et al., Nature 341: 149-152, (1989); Hohn, et al., Nature, 344: 339-341, (1990); Maisonpierre, et al., Science, 247: 1446-1451, (1990a); Rosenthal, et al., Neuron, 4: 767-773, (1990); Ernfors, et al., Proc. Natl. Acad. Sci., U.S.A., 87: 5454-5458 (1990); Jones and Reichardt,, Proc. Natl. Acad. Sci. U.S.A. 87: 8060-8064 (1990); Hallbook, et al., Neuron 6: 845-858, (1991); Ip, et al., Proc. Natl. Acad. Sci., U.S.A., 89: 3060-3064 (1992)]. This family of proteins plays an important role in both the developing and the adult vertebrate nervous system, where these proteins support neuronal survival.
Studies involving the in vivo actions of the neurotrophins, in particular the neurotrophin BDNF, have confirmed their actions in maintaining the survival and regulating the function or phenotype of various neuronal cells. Chronic intraseptal infusions of BDNF can prevent most of the axotomy-induced loss of cholinergic neuron staining in the medial septum [Morse et al, in press]. In otherwise intact rats, chronic infusions of BDNF above the substantia nigra elevate dopamine metabolism, as determined by increases in HVA concentrations in the ipsilateral caudate-putamen and even larger elevations in the DOPAC/dopamine and HVA/dopamine ratios [Altar, et al., In Vivo, Proc. Natl. Acad. Sci., (USA) (in press)]. A recent report described the in vitro survival effect of NT-3 and NT-4 (but not NGF) on locus ceruleus neurons (Friedman, et al. Exp. Neurol 119: 72-78 (1993).
There is also ample evidence of the presence of neurotrophin receptors in areas of the brain that are associated with depression. For example, high affinity binding sites for [125l]NT-3 are found within the medial substantia nigra and ventral tegmental area, nucleus accumbens, caudate-putamen, and raphe nucleus, and the binding to these sites is potently displaced by BDNF [Altar, et al., Am. Acad. Neurol. San Diego, Calif., (1992)]. BDNF mRNA is also present in these areas and appears to overlap with TOH-positive cells [Gall, et al., (1992)]. Intrastriatal injections of [125l]-labeled NT-3 or BDNF result in retrograde transport and accumulation of radioactivity within TOH-positive cells in these same regions, and intrastriatally injected [125l]-labelled BDNF or infused cold BDNF is transported to the median raphe area of the midbrain, which includes a large population of serotonergic neurons [Wiegand, et al., Soc. Neurosci. AB., 17: 1121, (1991)]. In brain sections, NT-4 binding has been found to be widely distributed throughout the brain including the cortex, striatum, hippocampus, cerebellum, olfactory bulbs, periaqueductal gray, and raphe nuclei.
Based on the in vitro activities as well as in vivo binding data, it is expected that the actions of BDNF, NT-3 and NT-4 will extend to brain regions containing or innervated by these neurons.
As described in copending Ser. No. 07/944,823 filed on Sep. 14, 1992, which in incorporated by reference herein, several members of the neurotrophin family have analgesic properties that may be due to their ability to alter serotonin neurotransmission within the brain or spinal cord. Infusions of BDNF and NT-3 in the area of the raphe nucleus resulted in elevated levels of serotonin, which may subsequently effect release of enkephalin or other naturally occurring opioids, thus causing the analgesia observed following their infusion. The effect of this alteration in serotonin metabolism on psychiatric disorders has not, to date, been reported.